The present invention is related to optical communication, and more particularly to wireless communications using infrared (IR) wavelength transmitters and receivers.
IR communication is often favored over radio frequency (RF) wireless communication for relatively short distances because of its immunity to most sources of noise. The IR signals are also easily contained within a room, as they do not penetrate walls. This limitation is actually a significant advantage in multi-dwelling buildings such as hotels and apartment buildings, as the signal from one room will not interfere with the signals in another room
One popular application for IR communication within rooms is for the control of appliances, such as TV and lights. Other applications include the exchange of data for burglar alarms system, access control systems, temperature controls and the like. In most of these applications, it is advantageous to work in the non-directional, diffuse mode of communications, where the link is not limited to line-of-sight alignment between the components that participate in the communication (the network nodes). Such systems depend on bouncing the IR signals from walls and ceiling, thus establishing a link between the nodes.
Due to the significant attenuation of IR signals with each bounce and with distance, diffuse mode communication requires high power levels of IR transmission and high sensitivity on the IR receiver side. The power that can be applied to the IR transmitter is limited by the rating of commercially affordable IR LEDs and safety considerations that preclude the use of lasers. It is thus of significant benefit to obtain high sensitivity IR receivers.
The theoretical limits on the ability of a receiver to retrieve a useable signal are determined by the ratio of the signal's strength to the strength of the prevailing noise (Signal to Noise Ratio, or SNR). In IR systems, the dominant noise is caused by ambient light sources such as the sun and incandescent and fluorescent lamps. Noise generated by close-by fluorescent lamps is intrusive for IR signal reception, as it presents high levels of energy in the optical wavelength of the IR signal. Fluorescent lamps that use electronic high-frequency ballasts are particularly troublesome, as they generate IR noise with modulation characteristics that are similar to the modulation schemes found to be most effective for IR communication networks.
One known method of increasing the sensitivity of a receiver is through the use of automatic gain control (AGC). For example, U.S. Pat. No. 1,719,845 to Martin teaches the use of AGC to compensate for weak radio signals. The amplitude of the detected RF signal is used to change the gain of the RF amplifier so that, when a smaller signal is received, the gain of the amplifier is increased to materially compensate for the smaller signal.
U.S. Pat. No. 4,216,430 to Kiyoshi describes an amplifier where the gain is controlled by the amplitude of the noise signal being received. A band-pass filter separates pulse noises from the normal audio signal. When the separated noise signal exceeds a preset threshold, the gain of the amplifier is momentarily reduced to prevent the noise from creating objectionable output.
U.S. Pat. No. 5,036,527 to Halim provides an AGC method where the gain of the amplifier is changed in digital steps based on the level of the signal. The incoming signal is compared against two fixed thresholds, and the gain is incremented or decremented depending whether the signal is larger than the higher threshold or smaller than the lower threshold.
U.S. Pat. No. 5,329,243 to Tay separates noise from signal using band-pass filter, and reduces the dynamic range of the AGC if a high level of noise is detected to prevent the noise from being amplified to an annoying level. The frequency response of the amplifier is also altered to attenuate high-frequency noise under these conditions.
U.S. Pat. No. 6,112,119 to Schuelke et al discloses an AGC method useable in amplifiers for sensing cardiac events. Due to the rhythmic nature of the expected signals, the gain is increased when an excessive period of time has elapsed without receiving cardiac signals.
The above examples of AGC provide methods to reduce the amplitude of the signal, or of the noise, which is mixed in with the signal, based on the amplitude of the incoming signal or noise. Where noise amplitude is used as the criterion for the gain setting, the noise is separated from the incoming signal through its frequency characteristics that allow the circuitry to distinguish noise from signal. For example, in the case of the Kiyoshi patent, a band-pass filter is used for the separation.
However, in IR communication systems used in the vicinity of man-made light sources, the dominant noise source is shot noise. Shot noise is white noise containing constant noise power per hertz of bandwidth. Such noise can not be readily distinguished from the data by its physical attributes, as the most common modulation techniques used for IR communication use pulse position modulation (PPM) or variants of that method. Data encoded in PPM appears with the same time and frequency characteristics as the peaks of shot noise. If the shot noise is present in strong enough amplitude, it can effectively block communications of the IR system.
The presence of some shot noise in a PPM data signal can be filtered using software algorithms. U.S. Pat. No. 5,128,792 to Teich titled “Self-Synchronizing IR Communication System” describes the use of a special PPM code that allows the IR receiver to recognize a packet of data even if it includes noise. The limitation of the Teich system is that it encumbers the decoding function of the receiver. The microprocessor in the receiver requires a large program and resources, while the data throughput is only a fraction what a non-noise-resistant code would provide with the same bandwidth.